Current waveguide technologies exhibit losses on the order of 100-200 dB/m the causes of loss are myriad, and include insertion losses, presence of impurities in the waveguide, bend losses, scattering losses caused by sidewall surface roughness, etc.
For example, the scattering of light from rough sidewall surfaces in semiconductor optical rib waveguides is a primary source of wave propagation loss. Photolithography and anisotropic dry-etching fabrication processes transfer photo mask line edge roughness and may introduce additional roughness to the etched sidewalls used to define the lateral effective index contrast, Δηeff, required to confine an optical mode. Theoretical and experimental studies of scattering loss α show that α typically scales with (Δηeff)2 or (Δηeff)3, σ2, and 1/d4, where σ and d represent the root-mean-squared surface roughness and waveguide horizontal dimension, respectively. To achieve high-density photonic integration, a high index contrast (HIC) is required to achieve small radius of curvature bent waveguides with low bend loss, which in turn necessitates a reduction in d to maintain single mode operation. Given the clear fundamental tradeoff between reduced bend size and increased scattering loss, minimizing the sidewall roughness σ becomes perhaps the most critical challenge to achieving a viable photonic integration technology.